1. Field of the Invention
The invention relates to the separation of toluene diisocyanate and/or higher boiling solvents from distillation residues by evaporating the residue in a fluidized bed at temperatures of 140.degree. C. to 280.degree. C. The distillation residues are produced during the preparation of toluene diisocyanate by the phosgenation of toluene diamine.
2. Description of the Prior Art
Ullmann's Encyclopedia of Technical Chemistry, 4th Edition (1977), Vol. 13, p. 351, discloses the reaction of toluene diamine with phosgene to produce toluene diisocyanate in the presence of an organic solvent, such as monochlorobenzene or o-dichlorobenzene which boils at a lower temperature than toluene diisocyanate. The use of solvents boiling at higher temperatures then toluene diisocyanate has also been disclosed. These solvents include methyldiphenylmethane, tetrahydronaphthylene, or alkylisophthalate (the latter in French Pat. No. 1 476 755). The phosgenation is generally carried out in two stages, initially at temperatures of 0.degree. C. to 100.degree. C. and subsequently at temperatures from 150.degree. C. to 180.degree. C. The residual phosgene is then removed from the reaction solution by distillation or by passing nitrogen through the solution. The reaction solution is then distilled into toluene diisocyanate, the solvent, and phosgenation by-products, which are primarily non-volatile residues.
If a solvent is used for the phosgenation, boiling lower than toluene diisocyanate, it is distilled off first during further processing. This leaves a crude toluene diisocyanate mixture which consists of 65 weight percent to 98 weight percent of toluene diisocyanate, 0 weight percent to 10 weight percent of lower-boiling solvent, and 2 weight percent to 25 weight percent of non-volatile residues. If a solvent is used boiling higher than toluene diisocyanate, the toluene diisocyanate alone or a mixture of toluene diisocyanate and part of the higher-boiling solvent is initially distilled off. This leaves a residue mixture 65 weight percent to 89 weight percent of higher-boiling solvents, 0 weight percent to 10 weight percent of residual toluene diisocyanate, and 2 weight percent to 25 weight percent of nonvolatile residues. The residues may be totally or partially dissolved or partially suspended.
The above-mentioned mixtures are now further concentrated by means of distillation in the first case to obtain the toluene diisocyanate and in the latter case, to reclaim the higher-boiling solvents. A distillation residue is obtained which still contains considerable quantities of toluene diisocyanate and/or higher-boiling solvents normally between 20 weight percent and 80 weight percent hereinafter sometimes referred to as desired product. Depending upon the manufacturing conditions, the residual components of the distillation residue primarily consist of 1 weight percent to 80 weight percent of urea compounds, 0 weight percent to 40 weight percent of uretdione, 0 weight percent to 60 weight percent of isocyanuric esters, 0.5 weight percent to 20 weight percent of carbodiimides, and 5 weight percent to 95 weight percent of higher-condensing or polymeric materials.
The reclaiming of toluene diisocyanate and/or the higher-boiling solvents is of primary importance for the profitability of the manufacturing process. Various processes were developed for reclaiming isocyanate from residue mixtures. German Patent Application No. 12 43 178, for instance, describes an extraction process. British Pat. No. 1 117 066 and German Patent Application No. 12 31 689 describe distillation processes. German Application No. 19 62 598 relates to a hydrolysis process with subsequent phosgenation of the hydrolysis products. A drawback of the above-mentioned processes is that large quantities of solvents are required for isolating the isocyanate. These solvents have to be purified in separate expensive process stages.
In order to avoid these drawbacks, the distillation residue may be removed by screw conveyors. The residue may be fed into a heated packed bed equipped with a screw agitator so that it descends when agitated together with the solids material in the vertical center section, loses the volatile components at the bottom, and moves upwards along the peripheral zones to form a dry solid residue (German Published Application No. 24 52 805). The latter process is for the separation of toluene diisocyanate as well as for higher-boiling solvents. Although these processes show rather favorable results, they also have certain drawbacks. One drawback, for instance, is that the equipment has moveable parts upon which the chlorine and chloride-containing distillation residues have a strongly corrosive effect.
It was known that packed materials can be dried and cooled with the aid of fluidized beds [Machinenmarkt, Wuerzburg, 18 (1975) 79, pages 1480 to 1973].